In the past, as illustrated in the above U.S. patent and applications, conventional copy-proof documents were comprised of a single tone cancellation phrase ("VOID", "COPY", etc.) and a single tone background pattern. Tone, in this context, refers to the dots or marks which cover a percentage of the printed area and have a frequency measured in dots, marks or lines per inch. The cancellation phrase was composed of a tone slightly larger in percentage, but significantly lower in frequency than the background tone.
Many different combinations were possible. For example, the cancellation phrase or void word could cover 22% (printed dot diameter of approximately 0.010 inches) with a frequency of 65 lines per inch. The background pattern in this instance could be 16% (printed dot diameter of approximately 0.005 inches) with a frequency of 133 lines per inch. Another example might use the same frequencies, but a void word composed of 15% (printed dot diameter of 0.008 inches) and a background of 10% (printed dot diameter of 0.003 inches).
The above techniques were successful and for properly printed documents the protection was excellent for normal settings of copiers such as the Xerox L-6500 color copier or similar equipment. However, protection was less than adequate for a full range of copier settings. At the lighter settings the resolution of the copier is such that it does not see the smaller background dots nor can it reproduce the larger dots that comprise the cancellation phrase. If the dot sizes are adjusted so that the cancellation phrase is visible at the lighter copier settings but not visible on the printed document, the background pattern becomes visible to the copier at darker settings. When this occurs the background pattern tends to obscure the cancellation phrase pattern.
The aesthetic quality of these documents is limited by three inherent weaknesses. First, the ability of screened tones to print detailed patterns. To fully reproduce a pattern with a screen tint, the pattern must be at least two line widths at its narrowest points. A 133 lines per inch tint can then carry print down to 0.0150 inches. The customary screen paired with it is a 65 line per inch tint. It can reproduce detail to only 0.030 inches. One can conclude that the quality of detail in a printed manuscript is limited by the size of the largest dots.
Secondly, the questionable accuracy in the frequency of screen tints can cause problems at the boundary between the cancellation phrase and the background. The most successful screen pair thus far has been the 65 and 133 lines per inch combination. As one can see, these are not exact multiples. Also, they are production quality screens and are at best accurate to .+-.1.5 lines per inch for the 65 lines per inch tint and .+-.3.0 lines per inch for the 133 lines per inch tint. This mismatch (as opposed to exact multiples) in frequency causes interference patterns at the boundaries. This is analogous to playing a chord on an out-of-tune piano; just as the tonal inaccuracies are audible, so are the boundaries visible.
Finally, even if the frequencies are matched as exact multiples (i.e. 65 and 130 lines per inch) the transition is still not hidden from the human perception. This can again be illustrated by a musical analogy. If we were to play a note on a piano, then play the same note but a step higher (a multiple of the original frequency) the transition is smooth. But even someone who is tone deaf can distinguish between the two frequencies. If we use a camouflage screen made up of a random pattern, or a very bold pattern, it tends to obscure the difference. But for a pattern that is not bold or is fairly regular, the transition in frequency becomes more noticeable.